1. Field of the Invention
The invention relates to a wireless communication system and a communication method for the wireless communication system and, more particularly, to a wireless communication system and communication method for the wireless communication system, which are usefully employed for a multichannel remote keyless entry (RKE) system, or the like.
2. Description of the Related Art
Generally, in a wireless communication system, such as the remote keyless entry system, that remotely operates a security device, or the like, through wireless communication, wireless intercommunication is carried out between a mobile device and a communication controller (in-vehicle device). The mobile device has a transmitting and receiving function and is held by a vehicle user. The communication controller (in-vehicle device) also has a transmitting and receiving function and is mounted on a vehicle. Then, through such intercommunication, a door lock of the vehicle is automatically locked or unlocked or a start of an engine is allowed.
Incidentally, such a wireless communication system mostly employs a so-called multichannel system for intercommunication between the mobile device and the in-vehicle device. That is, in this system, one of a plurality of prepared frequency bands is selectively used, and the frequency band used for intercommunication between the mobile device and the in-vehicle device is automatically changed each time the user operates the mobile device. By so doing, even when the intercommunication between the mobile device and the in-vehicle device using the selected frequency band is interfered because of deterioration of a communication environment due to noise, or the like, the frequency band used for the intercommunication is, where appropriate, changed from the frequency band that is significantly influenced by noise to the frequency band that is hardly influenced by noise.
However, when the frequency band (channel) used for the intercommunication between the mobile device and the in-vehicle device is changed each time the user operates the mobile device in this way, the frequency band can be changed in spite of a good communication environment between the mobile device and the in-vehicle device. That is, usually, the operation on the mobile device includes a “short press operation” of a push switch (button switch) provided for the mobile device, a “double short press operation” in which a short press operation is carried out twice within a predetermined period of time, and a “long press operation”. The in-vehicle device is configured to, for example, lock or unlock the door lock of the vehicle or start the engine on the basis of a determination as to these types of operation. Therefore, when the in-vehicle device receives a special switch operation, such as a “double short press operation” and a “long press operation”, the in-vehicle device changes the frequency band each time the user performs a switch operation. Eventually, this leads to a decrease in the reliability of communication between the mobile device and the in-vehicle device, for example, it is difficult to determine the type of switch operation, particularly, a special switch operation.
For this reason, it has been, for example, proposed that, when a special switch operation, such as a “double short press operation” and a “long press operation”, is performed, a frequency band used in an initial switch operation is stored and then the stored frequency band is used to carry out the following intercommunication based on the special switch operation, or the like. However, in this case, when the frequency band used for the “double short press operation”, the “long press operation”, or the like, that is, the stored frequency band, is interfered by noise, or the like, the frequency band is not reliably changed, so no intercommunication is established between the mobile device and the in-vehicle device.
Then, for example, Japanese Patent Application Publication No. 2001-103561 (JP-A-2001-103561) describes a wireless communication system that detects the receiving electric field intensity in each of a plurality of time slots used for time division multiplex communication between a transmitter and a receiver. Then, only when all the detected receiving electric field intensities in the time slots remain below a predetermined threshold continuously for a prescribed period of time or longer, it is determined that the communication environment is deteriorated and then the frequency band is changed. In this way, with the wireless communication system described in JP-A-2001-103561, the frequency band is changed only when it is determined that the communication environment is deteriorated, so there is no chance to unnecessarily change the frequency band used between the transmitter and the receiver. That is, even when various types of control are performed at the side of the in-vehicle device on the basis of the above described special operations (switch operations) on the mobile device, the high reliability of that communication, that is, the high capability of establishing communication, is also maintained.
Incidentally, in order to change the frequency band on the basis of all the detected receiving electric field intensities in the time slots used for the communication, especially, the time division multiplex communication, between a transmitter, such as a mobile device, and a receiver, such as an in-vehicle device, it is required to monitor all the receiving electric field intensities in the time slots used for those communications, so the configuration of the wireless communication system also becomes complex. That is, although it is intended to handle the above described special operations (switch operations) of the mobile device, the cost of the system for handling the special operations is not negligible, so there is still room for improvement in application to the remote keyless entry system, or the like.